Many electronic devices are fabricated by forming layers of material from various elements of the periodic table. For example, electronic devices such as photodetectors and heterojunction bipolar transistors are fabricated using various combinations of what are referred to as III-V semiconductor materials. These devices are typically formed having one or more p-n junctions or having p-type doped material and n-type doped material separated by a layer of intrinsic material. These devices are generally referred to as having a p-i-n junction. For example, in a photodetector, the intrinsic material forms the depletion region where light is converted to electron-hole pairs to generate an electrical signal proportional to the detected light. In some applications, such as in a mesa photodiode or heterojunction bipolar transistor, the intrinsic material or pn junctions are exposed at the device surface. These devices are very sensitive to leakage current in the intrinsic material or at the surfaces of the intrinsic material.
Devices in which the p-n junction is exposed, generally degrade rapidly due to environmental exposure unless they are enclosed in an expensive hermetic package. Examples of environment exposure include elevated temperatures in air during packaging (up to ˜300 degrees Celsius) or exposure to humidity during the course of normal operation. To provide environmental protection, a passivation layer is often formed over the exposed p-n junction or intrinsic material to prevent environmental exposure from degrading the performance of the device. Unfortunately, the oxide material that is native to the III-V semiconductor materials is unstable and gives rise to surface defects at the intrinsic layer-oxide interface when a passivation film using the native III-V oxide is formed over the intrinsic material layer. The surface defects formed at the intrinsic layer-oxide interface act as minority carrier recombination sites. The electron-hole pairs recombine at the minority carrier recombination site, thus preventing the electrical signal from flowing through the electrical contacts of the device. Instead, the recombination of the electron-hole pair at the defect site causes a leakage current to flow in the intrinsic material, thus degrading the performance of the device. In an HBT, these defects limit the performance of the transistor by limiting current flow through the junction. In a photodiode the optical response is reduced because the photogenerated electron-hole pairs do not flow through the external circuit.
Although passivation coatings have successfully been used in silicon wafer fabrication processing, the deposition of oxygen, silicon or hydrogen, which are typical components of a passivation film on a type III-V semiconductor material, causes defects to form at the semiconductor-passivation layer interface.